Gas turbine fuel control system for preventing compressor stall



2 Sheets-Sheet 1 RUSS ET AL CON- D. GAS TURBINE FUEL CONTROL SYSTEM FORPREVENTING COMPRESSOR STALL June 27, 1961 Filed Feb. 25, 1956 IN VEflTORS DANIEL s. ass.

A MES M. EASTMAN ATTORN E Y.

June 1951 D. 6. Russ ET AL 2,989,850

GAS TURBINE FUEL CONTROL SYSTEM FOR PREVENTING COMPRESSOR STALL FiledFeb. 25. 1956 2 Sheets-Sheet z -/OZ A DANIEL a 5%? 7 MES MIEASTMAN.

ATTORNEY.

2,989,850 GAS TURBINE FUEL CONTROL SYSTEM FOR PREVENTING COMPRESSORSTALL Daniel G. Russ, Hatboro, Pa., and James M. Eastman, South Bend,Ind., assignors to The Bendix Corporation,

a corporation of Delaware Filed Feb. 23, 1956, Ser. No. 567,261 Claims.(Cl. 60--39.28)

This invention relates to fuel feeding systems and more particularly toa fuel scheduling system for gas turbine engines.

Fuel systems for gas turbine engines normally must deal with two factorswhich limit fuel flow during acceleration. These are first, combustionchamber or turbine inlet temperature and second, compressor stall.Because of problems related to distribution errors and the difficulty ofobtaining sensitive, but durable, temperature sensing elements, it isconsidered desirable that these limits be avoided without measurement ofeither engine or turbine inlet temperature. An expression may be derivedfor fuel flow scheduled to give a constant maximum turbine inlettemperature. This expression is as follows:

W =6NF (N, r) where W =weight of fuel flow in pounds per hour6=compressor inlet total pressure corrected for standard conditions N=engine r.p.m.

r=compressor pressure ratio.

It has been demonstrated that compressor stall will be avoided if fuelis scheduled to the engine according to the equation W =6NF (r).

Each of the above expressions utilizes the same three engine operatingconditions; namely, 6, N, and r, and neither of these equations involvestemperature sensing. A control which meters according to these twoequations in such manner that the equation permitting the least fuel isalways overriding will schedule constant turbine temperatureaccelerations with overriding stall protection. It is, therefore, anobject of the present invention to provide a fuel control system whichwill utilize the above relations in mutually overriding fashion toschedule fuel. It is another object to provide a control system whichwill schedule fuel very close to temperature and stall limits of theengine without the necessity for actually sensing temperature. It is afurther object to provide a fuel feeding system which'will accomplishthe above objects and which requires that a minimum number of engineoperating conditions be sensed.

Other objects and advantages will become apparent from perusal of thefollowing specification taken in connection with the accompanyingdrawings, in which:

FIGURE 1 shows fuel requirements for a typical turboprop engine based onthe above equations and with the turbine inlet temperature establishedat a uniform maximum value; and

FIGURE 2 is a schematic drawing of a control which meters fuel accordingto the above equations.

--With reference to FIGURE 1, it will be seen that this drawing is agraph showing acceleration flow limits for a typical turbo-prop enginein which the vertical axis is plotted in terms of weight or fuel flowcorrected for speed and standard temperature conditions and thehorizontal'axis is plotted in terms of compressor pressure ratio. Whilethe present specification refers to compressor pressure ratio, this termwill be considered as including the ratio of any pressures sensed acrossany part of the compressor which will vary in substantially the samemanner as does the ratio across the entire compress'or... On this graphcurves a, b and c are all maximum 7 temperature lines for differentvalues of compressor inlet temperature. Curve d which intersects bothcurves a and b defines the boundary of the compressor stall area. Forall but very high temperatures at the compressor inlet, the stall areawill intersect the maximum temperature limit line. Curves e through edefine the relation of the corrected fuel flow to pressure ratio atspecific values of N. The family of curves e hence describes thecorrected fuel flow as the composite function (N, r). If the limitingvalue of Wg/(SN occurs when the curve e is reached and if curve e isshifted as shown when N is changed, then the values of will coincidewith those for curves 0, b and c, and the required limitation of fuelflow to limit turbine inlet temperature will be obtained.

A control which is to provide an optimum acceleration for an engine suchas that for which the curves of FIG- URE 1 were plotted must schedule atmaximum temperature up to the compressor stall area, follow the boundaryof the compressor stall area until maximum temperature or requestedspeed is reached, and then govern back to steady state fuel flow atmaximum speed. The present application is concerned entirely with anacceleration control and has not included means for speed governing,which might be accomplished by any of several wellknown methods. For thepurpose of discussing operation of the present device, it will beassumed that an allspeed governor is arranged to control a valvedownstream of valve 18.

From the foregoing it will be apparent that if the area A of a valve canbe made responsive to N and r such that the area varies as a function ofengine speed and compressor pressure ratio during accelerations in theregion where fuel how is limited by temperature and as a function ofcompressor pressure ratio only during the acceleration range over whichstall conditions may be encountered and that the head across the valvevaries directly as a function of 6 and engine speed, it should bepossible to meter fuel to the engine in accordance with the aboveequations. Such a control system is shown in FIGURE 2. In this controlsystem fuel is supplied from a source, not shown, through a conduit 10to a fluid pressurizing device shown herein as a variable displacementpump 12. Fuel from this pump is supplied through a conduit 14 to a headregulating system which controls the head across a metering valve 18.The axial position of valve 18 and, hence, its area is controlled by anactuator shown generally at numeral 20. The pump is of a conventionaldesign having a number of pistons 22 arranged to reciprocate in theirrespective chambers 24. A spring 26 in each one of these chambers urgesthe piston 22 to its maximum outward position. The pistons 22 haveimparted thereto a reciprocating motion through the action of a camplate 28 which is drivably attached to a shaft 30. This shaft isdirectly attached to the engine, not shown, and rotates at a speeddirectly proportional to engine speed. The number of reciprocationswhich each piston makes in a given period of time, then, will be definedby the speed of rotation of shaft 30. The amount of fuel which is pumpedduring each cycle of a plunger 22 is dependent upon the angle of the camplate 28. A shaft 32 drives this cam plate 28 and is positioned throughthe action of the forces exerted on each side of a piston 34 in achamber 36. As shown, the pressure on the bottom side of piston 34 ispump delivery pressure,

while working against this pressure on the top side of piston 34 is aservo pressure in chamber 36 and also the force exerted through theaction of a spring 38. An evacuated aneroid member 40 exposed to ambientpressures supplies a signal varying with changes in 6 to a conventionalservo pressure control valve 42 which, in turn, controls servo pressurein the chamber 36 by varying the effective flow area of a vent passage43 connected between the chamber 36 and a suitable source of drainpressure, not shown. The servo valve 42 by controlling the pressureabove piston 34 causes the piston to move in proportion with thepressure 6 so as to cause fuel displacement per pump revolution to varyin proportion with 6. It can be seen that the pump output varies inproportion with the product 6N.

The output of pump 12 is supplied through conduit 14 to a regulatorsystem 16 and to the main metering valve 18. Regulator system 16includes a fixed orifice 46 and chambers 48 and 50 communicating withthe fuel pressure upstream and downstream respectively of the fixedorifice. These will be referred to as P and P pressures, respectively.Immediately upstream of metering valve 18 is a chamber 52 containing acontrol pressure which will be referred to as P; pressure and downstreamof valve 18 a conduit 54 connects a chamber 56 with metered fuelpressure or R; pressure. Chambers 48 and 50 and chambers 52 and 56 areseparated by means of two equal area diaphragms 58 and 60, which arelightly loaded by two opposing springs 62 and 64. These diaphragms areeach connected to a shaft 66 carrying the regulator metering valveelements 68. By means of this arrangement the pressure difference (PP.;) across the control valve 18 is maintained at the value (P -P bycontrolling by-pass fuel which flows through a conduit 70 across valveelement 68 and through a conduit 72 to the inlet side of pump 12.Metered fuel crossing valve 18 is supplied to the engine, not shown,through a conduit 74.

The axial position of valve 18 is controlled by means of an actuator 20which has as a means of providing a speed input an engine driven shaft75 having mounted thereon a pair of governor weights 76. These weightsact to produce a translation of a shaft 78 which, in turn, carries anarm 80 having mounted on its extreme end thereof means for controllingthe flow through an orifice 82. Also supplied as an inlet to actuator 20is a servo pressure of comparatively high value which enters through aconduit 84. It will be observed that it is this high servo pressure asmodified by the action of a bleed 86 which is controlled through themovement of lever arm 80. This servo pressure is also supplied to achamber 88 where it acts to push a piston 90 in a leftward directionagainst the force of a spring 92 and also against a spring 94. A shaft96 is carried by piston 90 and seeks an axial position dependent uponthe balance of the forces exerted by springs 92 and 94, lever arm 80,and the fluid pressure in chamber 88. A cam 98 is anchored at one end tothe housing of actuator 20 and is pinned to shaft 96 in such manner asto rotate around its anchor point. The angular position of cam member 98is proportional to engine speed and by means of a contour ground on theface 100 of said cam any desired function of engine speed may beimparted.

High servo pressure from conduit 84 is also supplied through a fixedarea orifice 102 and a variable area orifice 104 to a central chamber106 which is in direct communication with the return side of the servopressure system via conduit 108. An intermediate control pressuredownstream of orifice 102 is supplied to a chamber 110 where it exerts aforce tending to push a piston 112 to the left against the action of aspring 114. Carried by piston 112 is a shaft 116 having positioned onthe left end thereof a roller 118. A shaft 120 is attached to roller 118and also to a roller 122. Roller 118 moves between a fixed track formedby part of the housing of actuator 20 and a tiltable bar or rod 124which is suspended from a bellows 126 responsive to compressor inletpressure by a link '128 and an additional bellows 130 responsive tocompressor discharge pressure by a link 132. =It will be observed thatlink 132 also acts to vary the effective area of orifice 104. Thisstructure acts to position shaft 120 as a function of compressorpressure ratio and operates as follows:

When the engine is at rest, the pressures on the exteriors of bellows126 and 130 are equal and the pull or upward forces exerted on theopposite ends of the rocking bar 124 are equal. The pressure in chamber110 being at this time relatively high due to the restriction of orifice104 by member 132, piston 112 is forced to the left thereby pushingroller '118 to its extreme lefthand position. As soon as the enginestarts, there is a build-up of compressor outlet pressure on the outsideof bellows 130. This causes the bellows to con tract which moves member132 away from orifice 104, allowing the pressure in chamber 110 toescape into chamber 106. As a result, spring 114 pushes piston 112, andhence, roller 118, to the right until its force is balanced by thepressure remaining in chamber 110. Since the upward forces exerted bybellows 130 and 126 are proportional to compressor inlet and outletpressures, respectively, the quotient or geometrical ratio of theseforces is proportional to compressor ratio. This positions roller 118and, hence, shaft 120 at a point of equilibrium for the then existingcompressor ratio.

Roller 122 rides on a tiltable bar or beam 134 which is attached at itsleft end to the housing of actuator 20 and supported on its other end bymeans of a spring 136. Attached to the right end of beam 134 is a link138 drivably attached to metering valve 18. Also pivoted on housing 20at the same point as beam 134 is a lever 140 having on one end thereof aroller 142 which rides on face 100 of cam 98 and on the opposite end acam face 144 arranged to exert a force against roller 122. Also arrangedto effect the position of roller 122 is a stationary cam 146. Cam 146has ground thereon a contour designed to limit the upward movement ofroller 122 and, hence, the upward movement of link 138 over thecompressor stall region so as to hold valve 18 to a limited area.Because of the movement of cam 98, lever 140 will tilt in relation tospeed and the cooperative movement of cam face 144 and shaft 120 acts toimpart to beam 134 and, hence, to link 138 a position varying with speedand compressor pressure ratio.

In considering the operation of the above device, it would be desirableto discuss a typical acceleration with reference to FIGURE 1. Considerfirst an engine operating at point A on the graph. This may beconsidered an idle condition, the pressure being at a very low value asis corrected fuel flow. The fuel flow is controlled by the governor andheld below the limit shown on FIGURE 1. Assume the pilot requests thegovernor to give maximum r.p.m. The entire fiow output of valve 18 willthen enter the engine. This will cause temperature and pressure ratio torise until point B on FIGURE 1 is reached. Note this is the flow whichgives the maximum permissible turbine inlet temperature. The engine nowaccelerates and r.p.m. and pressure ratio increase simultaneously todescribe an operating path along curve b until point C is reached. Itwill be remembered that curves a, b and c all represent corrected fuelflow for the same maximum temperature at different ambient temperatureconditions. For a hotter or colder day, then, point B might lie on acurve roughly parallel to any of a, b or c. As fuel flow increases frompoint A to point B, there would be some increase in the amount of fueldelivered by pump 12, and an increase in the speed of governor weight 76and, hence, a change in the angular position of cam 98. Between point Aand point B the roller 122 and actuator 20 is substantiallyat'itsextreme lefthand positionof travel. This position is efiected onlyslightly through the change in compressor pressure ratio and there isnegligible change in engine r.p.m. during this time. At point B thespeed factor which is introduced on cam 98 and the compressor ratiofactor on piston 112 become significant and roller 118 begins to operateto move roller 122 to the right. As roller 122 moves to the right, andcam 144 simultaneously moves clockwise, a gradually increasing rate offuel flow is permitted until roller 122 meets cam 146 which is thecompressor stall cam. Further movement to the right of roller 122 causesthe roller to be forced in a downward direction thus forcing beam 134and shaft 138 downwardly and reducing the elfective area of valve 18.The range of increase in fuel flow just described occurs from point B topoint C on FIG- URE 1 and the intersection of the temperature line withthe stall line occurs at point C. At point C roller 122 begins to followcam 146 and the fuel flow is reduced as will be seen by examination ofcurve d. As the danger of entering compressor stall decreases, the stalldam will allow an increased rate of fuel flow until flow is againequivalent to that permitted by the maximum temperature line b. Thisoccurs at point D on the graph. Fuel flow then continues at maximumtemperature until the speed governor brings the fuel flow tothe valuenecessary to support steady state operation at the desired increasedspeed. It will be observed that on curve b the compressor stall area didnot inter-pose a serious interruption in the fuel flow schedule. It was,however, necessary to interrupt curve b as shown from C to D in order toavoid running the engine into compressor stall. For colder days thisinterruption would be more severe as will be appreciated fromexamination ofcurvea I a Although 'only'one embodiment is shown herein,it is recognized that the teachings of this invention may take arnumber'of forms and modifications may be made to suit individual requirements.

We claim:

1. An acceleration limiting system for an engine having a compressor, acombustion, chamber, and a fuel conduit forconducting fuel to thecombustion chamber comprising a metering valve operative to cont-r01 thefuel flow through said conduit; a head regulator for controlling thepressure drop across said valve; a fluid pressure source for supplyingfuel to said conduit having means for varying its output with changes incompressor inlet pressure and engine speed; and means for controllingthe efiective area of said valve including a link attached to saidvalve; a pivotable bar operably connected to said link; a rolleroperatively connected to and movable along said bar to pivot the same; aservo-motor system for controlling the travel of said roller along saidbar including a fluid pressure responsive member exposed to compressorinlet pressure, a second fluid pressure responsive member exposed tocompressor discharge pressure, a member connected to each of saidpressure responsive members, a tiltable beam supported by both of saidmembers, a chamber, a source of fluid pressure in communication withsaid chamber, a piston in said chamber, a roller operatively connectedto and movable along said tiltable beam connected with said piston andwith said first named roller, and means controlled by one of said fluidpressure responsive means for varying the fluid pressure in saidchamber; a first cam and a second servo-motor system for controlling theposition of said cam including a chamber connected to said fluidpressure source, a piston in said chamber connected to said cam, aspring in said chamber opposing said fluid pressure, and speedresponsive means for varying the fluid pressure in said chamber; asecond cam actuatable by said first cam for modifying the position ofsaid first named roller over a portion of the acceleration range of saidengine; and a third cam for modifying the position of said 'roller overanother portion of the acceleration range of said engine.

2. An acceleration limiting system for an engine having a compressor, acombustion chamber, and a fuel con duit for conducting fuel to thecombustion chamber comprising a metering valve operative to control thefuel flow through said conduit, a head regulator for controlling thepressure drop across said valve, a fluid pressure source for supplyingfuel to said conduit having means for varying its output with changes incompressor inlet pressure and engine rotational speed, and means forcontrolling the eifective area of said valve including a link attachedto said valve, a pivotable bar operably connected to said link, a rolleroperatively connected to and movable along said bar to pivot the same, aservomotor system for controlling the travel of said roller along saidbar, means responsive to compressor pressure ratio for controlling saidservo-motor system, cam means, operatively connected to said roller formodifying the position of said roller during its travel along said bar,asecond servo-motor system for controlling the position of said cammeans and engine speed-responsive means operatively connected to saidsecond servo-motor system for controlling the operation of said secondservo-motor system and thus the position of said cam means as a functionof engine speed, and a cam member operatively connected to said rollerfor modifying the position of said roller during a portion of its travelalong said bar.

3. An acceleration limiting system for an engine as set forth in claim 2wherein said first named servo-motor system includes a fluid pressureresponsive member exposed to compressor inlet pressure, a second fluidpressure responsive member exposed to compressor discharge pressure,"amember connected to each of said pressure responsive members, a tiltablebeam supported by both of said members, a chamber, a source of fluidpressure in communication with said chamber, a piston in said chamber,-aroller tracking on said tiltable beam connected with said piston andwith said first named roller, and a means controlled by one of saidfluid pressure responsive means for varying the fluid pressure in saidcham-' her. i. 4. An acceleration limiting system for an engine as setforth in claim 2 wherein said second named servo-motor system includes achamber connected to a fluid pressure source, a piston in said chamberconnected to said cam means, a spring in said chamber opposing saidfluid pressure, and speed-responsive means for varying the fluidpressure in said chamber.

5. An acceleration limiting system for an engine having a compressor, acombustion chamber, and a fuel conduit for conducting fuel to thecombustion chamber comprising a metering valve operative to control thefuel flow through said conduit, a head regulator for controlling thepressure drop across said valve, a fluid pressure source for supplyingfuel to said conduit including means for producing a flow output varyingwith speed and compressor inlet presusre, and means for controlling theeffective area of said valve comprising a link attached to said valve,link actuating means including a roller, means responsive to compressorpressure ratio for positioning said roller, a first cam and a second camfor modifying the position of said roller, speed-responsive means forvarying the effective position of said first cam, and a linkage systemresponsive to movement of said roller for varying the position of saidlink.

6. An acceleration limiting system for an engine having a compressor, acombustion chamber, and a fuel conduit for conducting fuel to thecombustion chamber comprising a metering valve operative to control thefuel flow through said conduit, a head regulator for controlling thepressure drop across said valve, a fluid pressure source for supplyingfuel to said conduit including means for producing a flow output varyingwith speed and compressor inlet pressure, and means for controlling theeffective area of said valve comprising a link connected to said valve,a movable member operatively connected to said link, means responsive tocompressor pressure ratio for positioning said member, and first andsecond means for modifying the position of said member over portion ofits travel, one of said means being responsive to changes in speed ofsaid engine.

7. An acceleration limiting system for an engine having a compressor, acombustion chamber, and a fuel conduit for conducting fuel to thecombustion chamber comprising a metering valve operative to control thefuel flow through said conduit, a head regulator for controlling thepressure drop across said valve, a fluid pressure source for supplyingfuel to said conduit including means producing a flow output varyingwith speed and compressor inlet pressure, and means for controlling theeflfective area of saidvalve comprising linkage means drivably connectedto said valve, a movable member operatively connected to said linkagemeans, servo means responsive to compressor pressure ratio forpositioning said member, first cam means and second cam means formodifying the position of said member, said first cam means beingmovable in response to changes in speed of said engine, said second cammeans being fixed in position and operative to override said first cammeans during a portion of the travel of said movable member.

8. An acceleration limiting system for an engine having a compressor, acombustion chamber, and a fuel conduit for conducting fuel to thecombustion chamber comprising a metering valve operatively connected tosaid fuel conduit for controlling fuel flow therethrough, a fluidpressure source for supplying fuel to said conduit in such manner that,for a given area, the flow through said valve varies with engine speedand compressor inlet pressure, and means for controlling the eifectivearea of said valve including a link drivably connected to said valve, amovable member operatively connected to said link, means responsive tocompressor pressure ratio for positioning said member, and first andsecond means for modifying the position of said member over portions ofits travel, one of said means being responsive to changes in speed ofsaid engine and the other of said means being fixed in position.

9. An acceleration limiting system for an engine having a compressor, acombustion chamber, and a fuel conduit for conducting fuel to thecombustion chamber comprising a metering valve operatively connected tosaid conduit for controlling fuel flow therethrough, a fluid pressuresource for supplying fuel to said conduit in such manner that, for agiven area, the flow through said valve varies with engine speed andcompressor inlet pressure, and means for controlling the effective areaof said valve including a link drivably connected to said valve, amovable member operatively connected to said link, servo meansresponsive to compressor pressure ratio for positioning said member, afirst cam and a second cam for modifying the position of said member,said first cam being movable in response to changes in speed of saidengine.

10. An acceleration limiting system for an engine having a compressor, acombustion chamber, and a fuel conduit connected to conduct fuel to thecombustion chamber comprising a metering valve operatively connected tosaid conduit for controlling fuel flow therethrough, a fluid pressuresource for supplying fuel to said valve conduit in such manner that, fora given area of said valve, flow through said valve is proportional tothe product of engine speed and compressor inlet pressure and valveactuator means for controlling the effective area of said valve suchthat the area varies as a function of compressor pressure ratio andengine speed over a portion of the acceleration range of the engine andas a function of compressor pressure ratio over another portion of theacceleration range of the engine, said actuator means including firstmeans responsive to compressor pressure ratio, second means operativelyconnected to said first means and responsive to engine speed, and thirdmeans cngageable with said first means for overriding said second meansover the last mentioned portion of the acceleration range of the engine.

References Cited in the file of this patent UNITED STATES PATENTS2,644,513 Mock July 7, 1953 2,683,349 Lawrence July 13, 1954 2,688,842Oestrich et al. Sept. 14, 1954 2,691,268 Prentiss Oct. 12 ,19542,693,081 Russ Nov. 2, 1954 2,764,868 Watson et al. Oct. 2, 1956 UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,989,850 vJune 27 1961 Daniel G. Russ et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

. Column 6', line 18 after "means" strike out the comma; line 27 after"for" insert further -q Signed and sealed this 17th day of April 1962.

(SEAL) Attest:

ESTON e, JOHNSON I DAVID L. LADD ,Attesting Officer Commissioner ofPatents

